A number of products in the commodity chemicals industry, for example ammonia or methanol, are synthesized in gas catalysis on the industrial scale using fixed bed catalysts. The syntheses generally proceed with decreasing volume at very high working pressures at high working temperatures.
In the production of ammonia, the synthesis gas comprises hydrogen and nitrogen and optionally additionally inert gases such as methane and/or noble gases. In this process, the fresh synthesis gas is typically first compressed to a high pressure and the compressed synthesis gas is fed into a circuit which is guided through one or more catalyst-filled reactors in which ammonia is produced. A separation system has typically been provided in the circuit, with which the ammonia produced is withdrawn from the circuit in liquid form.
In industrial practice, large-scale syntheses that are typically executed as circulation syntheses, in the design as single-stream plants, however, are increasingly meeting limitations as a result of apparatuses, machines and pipelines. Proceeding, for example, from a maximum permissible working pressure of about 230 bara in the ammonia synthesis, economically viable construction limits for pressure vessels and pipelines are foreseeable. If the intention is to further increase the capacity of circulation syntheses without increasing the number of pressure apparatuses, technological alterations are necessary.
The ammonia formation reaction is a pronounced equilibrium reaction, the equilibrium position of which moves ever more to the side of the reactants with increasing temperature. In the region of the customary working temperatures for the catalyst beds and working pressures of about 200 bar, only ammonia equilibrium concentrations of 20-25% are achieved. According to the Le Chatelier principle, increasing the working pressure can move the equilibrium position toward higher ammonia concentrations. However, tight limits are placed on any increase in the synthesis pressure by the available compressors. Furthermore, the additional compressor work has an adverse effect on the process efficiency, and the higher working pressure results in greater wall thicknesses for the apparatuses and hence specifically higher production costs.
The ammonia formation reaction is highly exothermic, and so the temperature of the process gas as it passes through the catalyst bed increases constantly if no removal of heat is being undertaken. Ammonia synthesis converters with internal bed cooling have been proposed and indeed implemented in individual cases. However, they are a complex construction and correspondingly costly to manufacture. Moreover, the filling and withdrawal of the catalyst is much more complex than in the case of reactors without cooling internals in the beds.
Therefore the economic optimum has for several decades been considered to be the division of the total amount of catalyst between three individual beds and the re-cooling of the process gas by heat exchangers situated between the beds.
Various solutions have been proposed to date for increasing the capacity of existing plants in particular.
DE 100 57 863 A1 discloses a process for preparing ammonia from synthesis gas which, apart from the hydrogen and nitrogen reactants, contains inert constituents in at least two reaction systems, wherein ammonia is synthesized successively from synthesis gas in different synthesis systems, where ammonia is produced from a portion of the synthesis gas in each of the synthesis systems and a portion thereof is discharged.
DD 225 029 relates to a process for synthesizing ammonia from synthesis gas containing inert materials, which is characterized by the use of two reaction systems. In the first reaction system, fresh synthesis gas only is converted. The unconverted synthesis gas is guided together with the gas from a synthesis circuit to the second reaction system, where the further conversion to ammonia is effected.
It is also part of the prior art that, for energy reasons, the waste heat from the reaction is utilized immediately downstream of the ammonia reactor for production of steam or for boiler feed water preheating. The waste heat rises with the amount of ammonia produced and the circulation volume in the synthesis circuit.
U.S. Pat. No. 5,352,428 A discloses a process for preparing ammonia having two series-connected reaction apparatuses each comprising two catalyst beds. In this known process, the product gas stream is cooled down by means of a quench gas stream downstream of the first catalyst bed, through which a substream of the fresh feed gas is passed directly to the second catalyst bed. This measure leads to a reduced yield of product gas, since this quench gas stream used for cooling does not take part in the reaction in the first catalyst bed.
U.S. Pat. No. 7,683,099 B2 describes processes for conducting exothermic gas phase reactions with heterogeneous catalysis, in which two reaction apparatuses having a total of three catalyst beds are used, wherein two catalyst beds are disposed in one of the reaction apparatuses. This document also describes a variant in which a substream of the product gas, after passing through the second catalyst bed, is separated off and cooled significantly, and the ammonia is condensed out as product and removed from the plant. A substream of the highly cooled gas is then passed into a second reaction apparatus in which there is a third catalyst bed. A disadvantage of this procedure is, however, that the gas has to be heated up again before being introduced into the third catalyst bed in order to start the reaction, and so thermal energy is lost. Moreover, the capital costs for this variant are much higher, since the cooling and heating require additional equipment. Thus, the process according to U.S. Pat. No. 7,683,099 B2 is comparatively uneconomic.
However, the known processes and apparatuses for preparation of a product from synthesis gas are not satisfactory in every aspect, particularly owing to the increase in the number of pressure apparatuses for large-scale plants, and so there is a need for improved processes and apparatuses.
Thus a need exists for an advantageous process and an advantageous apparatus for preparation of ammonia from a synthesis gas, having the features stated at the outset.